The present invention is directed to measuring metal coatings on semiconductor wafer products, and more particularly, to measuring the thickness and sheet resistance of thin-film metal coatings on semiconductor wafer products using selectable calibration materials.
As semiconductor wafers increase in size, the costs involved in the production of these wafers also increase. The semiconductor fabrication industry continues to demand higher yields and shorter fabrication times, while insisting upon ever-increasing quality standards. A variety of inspection procedures have been employed during various stages of the semiconductor wafer fabrication process in an attempt to meet these demands. These inspection procedures include destructive, as well as nondestructive testing methods.
During a chemical mechanical polishing (CMP) process, for example, measurement of the thickness of a deposited metal layer on a semiconductor substrate is often required. Metal layer measurements may be performed off-line during a standalone process, or as part of the fabrication process.
In a destructive measuring process, a standard or electron microscope may be used to measure the thickness of a wafer""s coating after a cross-section has been obtained. When the thickness of a thin-film coating is greater than 10,000 xc3x85, this type of destructive measuring method often provides accurate measurements. However, measuring accuracy usually begins to degrade as the coating thickness falls below the 10,000 xc3x85 threshold.
Other types of measuring processes utilize sensitive eddy current sensors which do not destroy or significantly alter the article measured. Although eddy current sensors provide highly accurate readings, these sensors are susceptible to error. For example, the shifting of an electronic reference point due to thermal drifting often occurs at some point during the data collection and inspection process. To compensate for thermal drifting and to ensure accurate readings, the eddy current sensor must be constantly recalibrated.
While there have been attempts to employ highly accurate, nondestructive measuring devices to accurately determine the thickness of thin-film metal layers disposed on semiconductor substrates, improvement is still needed.
A method for identifying metal layer thickness of an inspection sample according to one embodiment utilizes an eddy current probe to obtain initial resistance and reactance measurements from the inspection sample. Once these measurements have been obtained, the relative distance between the eddy current probe and inspection sample is increased and terminating resistance and reactance measurements are obtained. An inspection sample intersecting line may then be calculated using the initial and terminating resistance and reactance measurements. An intersecting point between a natural intercepting curve and the inspection sample intersecting line may also be determined. A reactance voltage of the intersecting point along a digital calibration curve is calculated to identify a closest two of a plurality of calibration samples. The metal layer thickness of the inspection sample may then be calculated by performing an interpolation between the identified closest two calibration samples.
In accordance with one aspect of the present invention, each of the plurality of calibration samples have metal layers of a type of material that is different than the inspection sample.
In accordance with another aspect of the present invention, each of the plurality of calibration samples include a metal layer comprising annealed Ti 6-4, while the metal layer of the inspection sample comprises annealed copper.
In another aspect of the present invention, the inspection sample comprises a semiconductor wafer having a metal top-layer. In this aspect, each of the plurality of calibration samples includes a metal layer comprising annealed Ti 6-4, while the metal top-layer of the semiconductor wafer comprises annealed copper.
In still yet another aspect of the present invention, the inspection sample intersecting line eliminates offsetting error to compensate for thermal drift present in the eddy current probe.